Ethylene polymers have been used generally and widely as resin materials for various molded articles and are required to have different properties depending on the molding method and purpose. For example, polymers having relatively low molecular weights and narrow molecular weight distributions are suitable for articles molded by an injection molding method. On the other hand, polymers having relatively high molecular weights and broad molecular weight distributions are suitable for articles molded by blow molding or inflation molding. In many applications, medium-to-high molecular weight polyethylenes are desirable. Such polyethylenes have sufficient strength for applications which call for such strength (e.g., pipe applications), and simultaneously possess good processability characteristics.
Ethylene polymers having broad molecular weight distributions can be obtained by use of a chromium catalyst obtained by calcining a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere to activate it such that at least a portion of the carried chromium atoms is converted to hexavalent chromium atoms (Cr+6). This is commonly referred to in the art as the Phillips catalyst. The respective material is impregnated onto silica, fluidized and heated in the presence of oxygen to about 400° C.-860° C., converting chromium from the +3 oxidation state to the +6 oxidation state. A second chromium catalyst used for high density polyethylene applications consists of silylchromate (bis-triphenylsilyl chromate) absorbed on dehydrated silica and subsequently reduced with diethylaluminum ethoxide (DEALE). The resulting polyethylenes produced by each of these catalysts are different in some important properties. Chromium oxide-on-silica catalysts have good productivity (g PE/g catalyst), also measured by activity (g PE/g catalyst-hr) but produce polyethylenes with molecular weight distributions lower than that desired for certain applications. Silylchromate-based catalysts produce polyethylenes with desirable molecular weight distribution characteristics (broader molecular weight distribution with a high molecular weight shoulder on molecular weight distribution curve, indicative of two distinct molecular weight populations).
Monoi, in Japanese Patent 200202412 discloses the use of inorganic oxide-supported Cr+6-containing solid components (A) prepared by sintering under nonreducing conditions, dialkylaluminum functional group-containing alkoxides (B), and trialkylaluminum (C). The resulting ethylene polymers are said to possess good environmental stress crack resistance and good blow molding creep resistance. U.S. Application 2002042428 discloses a method of ethylene polymerization in co-presence of hydrogen using a trialkylaluminum compound-carried chromium catalyst (A), wherein the chromium catalyst is obtained by calcination-activating a Cr compound carried on an inorganic oxide carrier in a non-reducing atmospheric to convert Cr atoms into the hexavalent state and then treating A with a trialkylaluminum compound in an inert hydrocarbon solvent and removing the solvent in a short time.
Hasebe et al. Japanese Patent 2001294612 discloses catalysts containing inorganic oxide-supported Cr compounds calcined at 300° C.-1100° C. in a nonreducing atmosphere, R3-nAlLn (R=C1-12 alkyl; L=C1-8 alkoxy, phenoxy; 0<n<1), and Lewis base organic compounds. The catalysts are said to produce polyolefins with high molecular weight and narrow molecular weight distribution.
Hasebe et al., in Japanese Patent 2001198811 discloses polymerization of olefins using catalysts containing Cr oxides (supported on fire resistant compounds and activated by heating under nonreductive conditions) and R3-nAlLn (R=C1-6 alkyl; L=C1-8 alkoxy, phenoxy; n>0.5 but <1). Ethylene is polymerized in the presence of SiO2-supported CrO3 and a reaction product of a 0.9:1 MeOH-Et3Al mixture to give a polymer with melt index 0.18 g/10 min at 190° C. under 2.16-kg load and 1-hexene content 1.6 mg/g-polymer.
Da et al, in Chinese Patent 1214344 teaches a supported chromium-based catalyst for gas-phase polymerization of ethylene prepared by impregnating an inorganic oxide support having hydroxyl group on the surface with an inorganic chromium compound aqueous solution; drying in air; activating the particles in oxygen; and reducing the activated catalyst intermediate with an organic aluminum compound. 10 g commercial silica gel was mixed with 0.05 mol/L CrO3 aqueous solution, dried at 80-120° C. for 12 h, baked at 200° C. for 2 h and 600° C. for 4 h, reduced with 25% hexane solution of diethylethoxyaluminum to give powder catalyst with Cr content 0.25% and Al/Cr ratio of 3.
Durand et al, U.S. Pat. No. 5,075,395, teaches a process for elimination of the induction period in the polymerization of ethylene by bringing ethylene in contact under fluidized-bed polymerization conditions and/or stirred mechanically, with a charge powder in the presence of a catalyst comprising a chromium oxide compound associated with a granular support and activated by thermal treatment, this catalyst being used in the form of a prepolymer. The Durand process is characterized in that the charge powder employed is previously subjected to a treatment by contacting the said charge powder with an organoaluminum compound, in such a way that the polymerization starts up immediately after the contacting of the ethylene with the charge powder in the presence of the prepolymer.
McDaniel, in U.S. Pat. No. 4,559,394 teaches the polymerization of olefins using activated chromium catalysts and tertiary alcohols. These patents teach the addition of alcohols to chromium oxide to improve chromium distribution. McDaniel adds the tertiary alcohol prior to catalyst activation. Interestingly, McDaniel teaches that silanols do not work to achieve this end.
U.S. Pat. Nos. 4,454,242 and 4,451,573 to Ikegami et al, employ silanols in conjunction with chromium oxide catalysts treated with zirconium or titanium and alkylmagnesium compounds to make improved environmental stress crack resistance (ESCR) products.
Chromium catalysts based on chromocene and silanols have been prepared and deposited on silica to increase catalyst activity as taught in U.S. Pat. No. 4,153,576 to Karol et al. U.S. Pat. Nos. 3,767,635; 3,629,216; and 3,759,918, assigned to Mitsubishi Chemical Industries, Ltd., teach the addition of pentaalkylsiloxyalanes to supported chromium oxide catalysts to make useful polyethylenes.
Chromium oxide (CrOx) based catalysts have high activity with moderate induction times and produce polymers with high molecular weights and intermediate molecular weight distributions. Silylchromate-based catalysts have poorer activity, but produce polymers with a broader molecular weight distribution. Silylchromate catalysts are typically more costly than chromium oxide catalysts. It would be desirable to have a method that allows for the tuning of chromium oxide based catalysts such that the polymers produced by them approach the characteristics of polymers produced using silylchromate-based catalysts. For background information regarding silylchromate catalysis, see e.g., U.S. Pat. Nos. 3,324,095 and 3,324,101 to Carrick et al. The prior art lacks an inexpensive, facile method for modifying a chromium oxide catalyst such that polymer produced by it can be variably tuned to approach polymer produced by silylchromate-based catalyst systems. Additionally, the prior art is devoid of any teaching of the use of silanols in a two-catalyst system to obtain polymers with bimodal molecular weight distribution profiles.
While the prior art contains these and other examples of the use of modified Phillips-type catalysts, there has not yet been disclosed a method for the control of molecular weight distribution. The present invention provides a method for the production of polyethylene characterized by the control of both the molecular weight and the breadth of molecular weight distribution. The present invention also provides a method to produce a bimodal polyethylene through the use of two chromium-based catalyst systems.